Continuity mistake: When they are driving up the mountain to get to her kids, they cross a river and they get hit on the passenger side of the truck shattering the window, but later when they show them going up the mountain there is a new window there.
Factual error: That truck they're driving had better be fitted with a couple of powerful rocket motors, because they outdrive a pyroclastic explosion, which would be moving at about 300kmh. We see the pyroclastic cloud just metres behind the truck - it would overtake them in seconds.
Revealing mistake: After Dante's Peak explodes for the final time, the pyroclastic cloud rolls down the mountain toward the town. Harry shouts "Don't look back, kids, don't look back." and then we are shown a wide shot of the truck zooming down the ash-choked streets. Unfortunately, it's painfully obvious that this is either a miniature or CGI shot, because the truck isn't leaving any tracks behind it or trailing any clouds of ash despite its high speed. (01:28:19)
Plot hole: After the volcanic eruption, there's a scene where we see our heroes going across a lake, which has been turned to acid by the lava. If this acid is strong enough to corrode the propeller, then surely the fumes would be destroying their lungs, in which case they should be dead, or close to it.
Continuity mistake: When Rachel and Harry are crossing the river in the truck, they get obviously very wet. By the time they get to Ruth's lodge, they seem quite dried out.
Other mistake: The first time Rachel brings the team coffee, she has six coffees on a tray. There's five cappuccinos and one regular coffee for Harry. On the tray there is five coffees on one side and one coffee at the other side, obviously being Harry's regular coffee, so that Rachel would be able to tell the them apart. But when she gives Harry a coffee, she gives him one of the five standing together, so he obviously got one of the cappuccinos.
Other mistake: At the end when they are rescuing Harry from the mine, we see a helicopter land and kick up tons of volcanic ash. Given the destructive nature of ash to the engine (as seen in the earlier scene), it would not make any sense to have a helicopter operate anywhere near the area.
Factual error: In the scene where Paul and the National Guardsman are leaving the motel for the final time after packing up, Paul follows the two Humvee's out of town. Being a Guardsman myself, I know for a fact that all civilians would be in front and Humvees would be following them. It is standard operational procedure.
Revealing mistake: In the scene where the van is about to be flipped over on the washed out bridge, wires can be seen on the hood of the miniature that pulled it across.
Revealing mistake: When the volcano begins to erupt, cars are seen driving out of the city. When the motorway collapses, and the cars go with it, in the close up shots, there are no drivers inside them.
Continuity mistake: Just after the helicopter crashes, check out the two full-view shots of the vehicle that show the top running lights. Of the three that are left, the middle one has been knocked crooked. After these two shots, the lights are all perfectly aligned again.
Continuity mistake: Near the start of the movie, during the violent eruption when the major characters have quickly loaded their gear into the truck and are trying to escape, the shot changes back and forth from looking into the cabin of the truck from the front and then looking forward out through the windscreen. The windscreen wipers are running at very different speeds. From inside, they run very fast (probably so you can see out through the mud) and very slowly from outside (probably to accentuate the mud etc).
Deliberate mistake: Geologically, the type of volcano in the movie shouldn't erupt so much low-viscous magma; but due to make the volcano exciting to the audience, the movie makers probably deliberately ignored this mistake.
Factual error: The scene in which the main characters are in the ranger station truck and are driving across the lava flow, you see the tire getting absolutely covered in lava, flinging it inside the wheel well, plus the bottom of the truck is basically only an inch or two above it. How did the gas tank and the fuel lines survive that miraculously without melting or bursting? The heat would literally boil the fuel inside the lines and the truck would no longer be able to run.
Deliberate mistake: In the shot right after Harry successfully maneuvers out of the river, the truck he's driving should still have water pouring out of it. When he was initially pulling out, the water in the truck was still up to the bottom of the steering wheel, and we can see it draining out, but it would take much longer than a second or two for the water to fully drain out- the truck's completely dry and not leaking at all. (01:03:40)
Factual error: When inside the boat going across the lake turned to acid, they are moving at a high speed. But all a sudden when they lose the prop, the boat is at a complete stop and actually turning. 2 problems with that scene. The momentum of the boat at a high speed would have continued moving it forward for quite some time. It would not stop all a sudden. And when he is using his shirt to row the boat, the boat would have turned, because he was only paddling on one side.
Revealing mistake: During the pyroclastic flow scene, the flow sweeps a tractor trailer off an overpass, which blows off some of the side panels in the back. Watch the back of the truck before the flow hits it, the panels that come out are already popped out and ready to blow off.